Serum protein binding of weakly acidic drugs (e.g., penicillins, sulfonamides) is impaired in uremia; that of basic drugs (e.g., trimethoprim) is normal. The uremic binding defect is not corrected by hemodialysis or prolonged in vitro dialysis, but is corrected by successful renal transplantation or treatment of uremic sera with active charcoal at acid pH. The chemical nature of the compound(s) responsible for defective protein binding in uremia is at present unknown. Extraction of uremic sera at acid (pH 3.0) with an organic solvent (n-butyl chloride) or treatment with anion exchange resin fully corrected binding defects for acid drugs; binding of basic drugs was unchanged. Fractionation of the organic solvent layer with purified human serum albumin gave a homogeneous fraction on thin layer chromatography which could induce binding defects when added to normal human sera. The resin eluate was also shown to contain the substance which was able to induce similar defects in normal human sera. This factor is a dialyzable and apparently weakly acidic compound with a molecular weight less than 500. It is lipid soluble and tightly bound to albumin at physiologic pH, but extractable at acidic pH. The objective of this proposal is to chemically define the structure of the binding defect inducer(s) and to relate the structure of the compound(s) to the genesis of drug binding defects in uremia at a molecular level. The kinetics involved in the interaction between the binding defect inducer(s) and the albumin molecule as well as its effect on the antimicrobial activity of antibiotics and changes in free drug concentration of other pharmacological agents will also be investigated.